Asteroids

Near-Earth Asteroids.

David Tholen is tracking near-Earth asteroids to help determine which of them may represent an impact hazard to Earth.

One object in particular has become of significant interest to the impact hazard community. Asteroid 2011 AG5 is an approximately 150 meters in diameter with a 1 in 550 chance of impacting Earth in 2040. If it becomes necessary to deflect this object to prevent an impact, it would far easier to do so before its next close approach to Earth in 2023, which leaves little time to prepare such a mission.

Tholen’s group has also measured the density of a small near-Earth asteroid, 2009 BD, via the detection of radiation pressure effects acting on the object. The results indicate a nominal density that is less than that of water, suggesting a very porous internal structure, which has also been seen in a couple of other small asteroids. If these results can be shown to apply to larger near-Earth asteroids, the potential impact energy estimates could be revised downward, making them less of a threat.

Tholen was also a member of the imaging team for the Japanese Hayabusa mission, which returned samples from the near-Earth asteroid Itokawa . The target surprised many with its lack of obvious craters and its boulder-rich surface. The images are suggestive of a rubble pile interior structure.

Near-Earth Asteroid Itokawa

Minimoons

Some near-Earth asteroids can be temporarily captured by the Earth's gravity if they have a slow relative speed while passing through the Earth-Sun Lagrangian points. Robert Jedicke has been calculating the population characteristics of these irregular natural Earth satellites.

He finds that any given time there should be at least one such object of at least 1 meter diameter orbiting Earth. His results are consistent with the single known natural minimoon, 2006 RH120, a few-meter diameter object that was captured for about a year starting in June 2006. Jedicke estimates that about 0.1% of all meteors impacting the Earth were once minimoons.

Main-Belt Asteroids

Some of the oldest materials in the solar system are the refractory minerals contained in calcium, aluminum-rich inclusions that are found in chondritic meteorites.

Bobby Bus has been using infrared spectroscopy to estimate the concentration of these minerals in a number of main-belt asteroids. He has found several asteroids with concentrations of these minerals 2-3 times higher than in any meteorite. These asteroids must therefore be older than any known sample in our meteorite collections, making them prime candidates for future sample return missions.

Three main-belt, calcium-aIuminum-rich asteroids showing the 2-micron absorption band diagnostic of aluminous spinel.